---
title: Linting code for LLMs with tree-sitter
excerpt: Aider now lints code after every LLM edit and automatically fixes errors, using tree-sitter and AST-aware code context.
highlight_image: /assets/linting.jpg
nav_exclude: true
---
{% if page.date %}
<p class="post-date">{{ page.date | date: "%B %d, %Y" }}</p>
{% endif %}

[![Linting code](/assets/linting.jpg)](https://aider.chat/assets/linting.jpg)

# Linting code for LLMs with tree-sitter

Aider now lints your code after every LLM edit, and offers to automatically fix
any linting errors.
You can also use aider's lint-and-fix functionality on your source files any time
you like, to speedily resolve issues with code written by humans.

Aider shows linting errors to the LLM in a novel format,
using tree-sitter
to help display relevant code context for each
error.
This increases the ability of the LLM to understand the problem and
make the correct changes to resolve it.

Aider ships with basic linters built with tree-sitter that support
[most popular programming languages](https://github.com/paul-gauthier/grep-ast/blob/main/grep_ast/parsers.py).
These built in linters will detect syntax errors and other fatal problems with the code.

You can also configure aider to use your preferred linters.
This allows aider to check for a larger class of problems, keep the code style
aligned with the rest of your team, etc.

## Linting and fixing your code

Aider now lints each source file after it applies the edits
suggested by an LLM.
If problems are found, aider will ask if you'd like it to
attempt to fix the errors.
If so, aider will send the LLM a report of the lint errors
and request changes to fix them. This process may iterate a few times
as the LLM works to fully resolve all the issues.

You can also lint and fix files any time, on demand from within the aider chat or via the
command line:

- The in-chat `/lint` command will lint and fix all the files which have
been added to the chat by default. Or you can name any files
in your git repo as arguments.
- From the command line, you can run `aider --lint` to lint and fix
all the dirty files in the repo.
Or you can specify specific filenames on the command line.


## An LLM-friendly lint report

Most linting tools produce terse and cryptic output,
which is one reason many engineers appreciate IDEs that highlight
linting errors.
LLM's don't have the luxury of using an IDE, so aider sends
the linting errors in an LLM friendly format.

Here's an example of raw output of the `flake8` python linter:

```
app.py:23:36: F821 undefined name 'num'
app.py:41:16: F541 f-string is missing placeholders
```

This sort of output depends on the user to reference line numbers to find and fix
each reported error.
LLMs are quite bad at working with source code line numbers, often
making off-by-one errors and other mistakes even when provided with
a fully numbered code listing.

Aider augments the raw linter by
displaying and
highlighting the lines that have errors within their
containing functions, methods, classes.
To do this, aider uses tree-sitter to obtain the code's AST and analyzes it
in light of the linting errors.
LLMs are more effective at editing code that's provided
with context like this.

```
app.py:23:36: F821 undefined name 'num'
app.py:41:16: F541 f-string is missing placeholders

app.py:
...⋮...
  6│class LongNum:
  7│    def __init__(self, num):
  8│        """
  9│        Initialize the number.
 10│        """
...⋮...
 19│    def __str__(self):
 20│        """
 21│        Render the number as a string.
 22│        """
 23█        return str(num)
 24│
 25│
 26│@app.route('/subtract/<int:x>/<int:y>')
...⋮...
 38│@app.route('/divide/<int:x>/<int:y>')
 39│def divide(x, y):
 40│    if y == 0:
 41█        return f"Error: Cannot divide by zero"
 42│    else:
 43│        result = x / y
 44│        return str(result)
 45│
...⋮...
```

## Basic linters for most popular languages

Aider comes batteries-included with built in linters for
[most popular programming languages](https://github.com/paul-gauthier/grep-ast/blob/main/grep_ast/parsers.py).
This provides wide support for linting without requiring
users to manually install a linter and configure it to work with aider.

Aider's built in language-agnostic linter uses tree-sitter to parse
the AST of each file.
When tree-sitter encounters a syntax error or other fatal issue
parsing a source file, it inserts an AST node with type `ERROR`.
Aider simply uses these `ERROR` nodes to identify all the lines
with syntax or other types of fatal error, and displays
them in the LLM friendly format described above.

## Configuring your preferred linters

You can optionally configure aider to use
your preferred linters with the `--lint-cmd` switch.

```
# To lint javascript with jslint
aider --lint-cmd javascript:jslint

# To lint python with flake8 using some specific args:
aider --lint-cmd "python:flake8 --select=E9,F821,F823..."
```

You can provide multiple `--lint-cmd` switches
to set linters for various languages.
You can also durably set linters in your `.aider.conf.yml` file.

